In the case of effectively using a flammable gas, it is necessary to separate a gas such as air from a source gas containing the flammable gas, and concentrate the flammable gas to an appropriate concentration range. Various such apparatuses and methods for concentrating a flammable gas have been proposed, and inventions have been proposed in which a gas (so-called coal mine gas) generating from a coal mine in the form of a methane-containing gas is used as a source gas, air (mainly containing nitrogen, oxygen, and carbon dioxide) is separated from the source gas by using an adsorbent, and methane is concentrated for use (Patent Document 1, for example).
More specifically, Patent Document 1 proposes an invention of an apparatus and a method for concentrating methane. According to the invention, with the use of natural zeolite, which has a very slower adsorption rate for methane than for nitrogen, as an adsorbent (i.e., with the use of an adsorbent that preferentially adsorbs miscellaneous gases such as nitrogen, oxygen, and carbon dioxide to methane), coal mine gas is introduced into an adsorption tower filled with the adsorbent by a compressor or the like until a predetermined pressure is reached. Then, the miscellaneous gases that are contained in the coal mine gas are adsorbed first at the front portion (lower portion) of the adsorption tower, and methane, for which the adsorption rate is slow, is adsorbed at the back portion (upper portion) of the adsorption tower. Further, the methane is released from the upper portion of the adsorption tower until it reaches atmospheric pressure.
Thereby, the air can be separated from the coal mine gas as the source gas by using the adsorbent, the methane can be concentrated, and the concentrated methane can be used as a fuel or the like.
That is, as the PSA cycle, a configuration is conceivable that is provided with a control means for successively performing an adsorption process of supplying the source gas to the adsorption tower, adsorbing the miscellaneous gases on the adsorbent in the adsorption tower, and releasing methane, and a desorption process of desorbing the miscellaneous gases from the adsorbent in the adsorption tower so as to regenerate the adsorbent in the adsorption tower.
With regard to such a PSA cycle, it is desired to perform a pressure equalization process of transferring the gas in an adsorption tower in which the adsorption process has been finished and which is in the high pressure state into another adsorption tower that is in a lower pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, and a pressure equalization process of receiving, after finishing the decompression process, the gas from another adsorption tower that is in a higher pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, thereby improving the efficiency of the energy required for pressure increase and pressure lowering in the adsorption towers, and also improving the recovery rate of a gas to be purified, while improving the purity of the gas to be purified.
Note that in the present invention, of a pair of adsorption towers between which the pressure equalization process is performed, the pressure equalization process performed in the adsorption tower whose internal pressure is lowered by transferring the gas to the other tower is referred to as the pressure equalization (pressure lowering) process, and the pressure equalization process performed in the adsorption tower whose internal pressure is increased by receiving the gas from the other tower is referred to as the pressure equalization (pressure increasing) process.